The present invention relates to a high definition digital television and more particularly to a pattern-adaptive digital comb filter which separates luminance and color signals by adaptive comb filtering according to the correlation between upper and lower horizontal scanning lines of three successive horizontal lines, when the luminance and color signals from an input video signal are to be separated.
In a conventional color TV, the process of separating the input video signal into the luminance and color signal is essential, and this process is carried out by a conventional comb filter.
In digital color televisions such as ID(Improved Definition) and ED(Extended Definition) televisions, in particular, which require high definition, the efficient separation of the luminance and color signals is required.
The comb filter is used on an assumption that the phase of color signal is inverted every 1 H(one horizontal scanning period. That is, the interleaving of the video signal in NTSC(National Television System Committee) method corresponds to this.
Also, the signals before and after 1 H must be similar.
FIGS. 1 (A) and (B) are conventional circuits for separating the luminance and color signals.
A circuit shown in FIG. 1 (A) is largely divided into two parts: One part is a 2 H comb filter 1, including first and second 1 H delays 1a and 1b, an adder 1c, first and second half-amplifiers 1d, and 1f, and a subtracter 1e, and the other part is an output stage 100 for providing the luminance and color signals, including a band pass filter (BPF) 102, a delay 104, and a subtracter 106.
Referring to FIG. 1 (A), when a digital video signal is applied through an input port 101, the digital video signal is delayed by 1 H in the first 1 H delay, 1a, and is subsequently applied to the second 1 H delay, 1b, the adder 1e, and the delay 104, in the output stage 100.
The second 1 H delay, 1b, delays the 1 H-delayed video signal once more, by 1 H, to provide a 2 H-delayed video signal to the adder 1C.
Then, the adder 1c adds the 2 H-delayed video signal to the input video signal. That is, a new video signal, in which luminance and color signal levels are added to each other by addition of two video signals with the same phase, appears at the output terminal of the adder 1c and is provided to the first half-amplifier 1d.
At this time, the first half-amplifier 1d decreases the level of the new video signal by half.
Thus, the video signal with an averaged level of a present input video signal V and a 2 H-delayed video signal are applied to the subtracter 1e. The subtracter 1e subtracts the averaged video signal from the 1 H-delayed video signal VH to remove the luminance signal components.
In the NTSC method, on the other hand, since the phases of the video signals before and after 1 H are inverse to each other, the phases of the color signals of the 2 H-delayed and the present input video signals are the same, while the phases of the color signals of the 1 H-delayed video signal VH and the 2 H-delayed video signal VHH, or the phases of the color signals of the 1 H-delayed and the present input video signals, are inverse.
Subtracting the averaged video signal from the 1 H-delayed video signal VH under an assumption that the luminance signal levels of the video signals before and after 1 H are almost equal to each other, the luminance signal components are cancelled, and only the sum of the color signal levels appears at the output terminal of the subtracter 1e.
Therefore, the second half-amplifier 1f provides the averaged value of the color signal levels of the 2 H-delayed video VHH and the present input video signal, by decreasing the sum of the color signal levels by half.
The output of the second half-amplifier 1f includes the color signal components filtered in a vertical direction and a complete color signal C is finally provided from the BPF 102.
At this time, the BFF 102 passes only the signal within color signal band in a horizontal direction.
Also, the delay 104 delays the 1 H-delayed video signal VH by a time that corresponds to the delayed time, until the generation of the complete color signal C passes through the aforementioned circuits to provide the 1 H-delayed video signal VH in synchronization with the color signal C.
Then, the subtractor 106 separates and provides only the luminance signal Y, by subtracting the color signal C from the 1 H-delayed video signal.
Therefore, the input digital video signal is separated into the digital luminance signal Y and color signal C.
At this time, the comb-filtering performed by the 2 H comb filter 1 means the separation of the color signal components, from the input video signal and the delayed video signal, by a predetermined time, while cancelling the luminance signal components.
As described above, the circuit shown in FIG. 1(A) can separate the luminance and color signals from the input video signal.
But, if the vertical transition occurs among the three successive horizontal scanning lines, the artifacts such as cross color and dot disturbance are generated in the circuit shown in FIG. 1 (A).
FIG. 1 (B) is another separation circuit of luminance and color signals comprising an adaptive 2 H comb filter.
Referring to FIG. 1 (B), a 1 H comb filter 2 comprises first and second 1 H delays 2a and 2b, subtracters 2c and 2d, and first and second half-amplifiers 2e and 2f.
An adaptive selection circuit 3 also comprises first.about.third low pass filters (LPF) 3a.about.3c, subtracter 3d and 3e, first and second absolute value detector 3f and 3g and a correlation detector 3h.
Finally, an output stage for providing the luminance and color signals comprises a band pass filter (BPF) 102, a delay 104, and a sutracter 106.
In the circuit shown in FIG. 1 (B), if a digital video signal is applied through an input port 101, the input video signal is delay by 1 H in the first 1 H delay 2a and is subsequently applied to the second 1 H delay 2b, the subtracters 2c and 2d, the second LPF 3b, and the delay 104.
The second 1 H delay 2b delays the 1 H-delayed video signal once more by 1 H, to provide a 2 H-delayed video signal to the subtracter 2d and the third LPF 3c.
Therefore, the subtracter 2c subtracts the 1 H-delayed video signal VH from the input video signal V, through the input port 101. That is, the luminance signal components are cancelled by subtracting the 1 H-delayed video signal VH from the input video signal V, while the color signal components are added, since the phases of the color signals of the 1 H-delayed video signal and the present input video signal are inverse to each other and their luminance signals almost equal each other.
Then, the sum of the color signal component levels is provided to the first half-amplifier 2e.
On the other hand, the subtracter 2d subtracts the 1 H-delayed video signal VH from the 2 H-delayed video signal VHH.
Similarly, the luminance signal components are cancelled, while the levels of the color signal component are added, since the phases of the color signals of the 1 H-delayed video signal VH and the 2 H-delayed video signal VHH are inverse to each other and their luminance signal components almost equal to each other.
Therefore, the sum of the color signal components is provided from the subtracter 2d to the second half-amplifier 2f.
The first half-amplifier 2e provides a first averaged color signal component of the 1 H-delay video signal VH and the present input video signal V to an input port Io of a multiplexer 4 by decreasing the added color signal component of the subtracter 2c by a half.
Also, the second half-amplifier 2f provides a second averaged color signal component of the 1 H-delayed video signal to another input port Ii of the multiplexer 4, by decreasing the added color signal component in the subtracter 2d by a half.
On the other hand, the first.about.third LPF's 3a.about.3c filter the present input video signal V, the 1 H-delayed video signal VH, and the 2 H-delayed video signal VHH, to provide each luminance signal.
The subtracter 3d subtracts the output of the first LPF 3a from the output of the second LPF 3b and provides the result to the first absolute value detector 3f, while the subtracter 3e subtracts the output of the third LPF 3c from the output of the second LPF 3b and provides the result to the second absolute value detector 3g.
Then, the first absolute value detector 3f detects the absolute value of the output of the first subtracter 3d and provides the result to an input terminal of the correlation detector 3h, while the second absolute value detector 3g detects, the absolute value of the output of the second subtracter 3e and provides the result to another input port of the correlation detector 3h.
Therefore, the correlation detector 3h detects the correlation between the outputs of the two absolute value detectors 3f and 3g and provides a logic selection signal according to the detected degree of correlation to a selection signal input port S of the multiplexer 4.
At this time, the correlation detector 3h provides the selection signal so that the multiplexer 4 selects and provides the color signal component with a larger degree of correlation, that is, the comb-filtered color signal component corresponding to a smaller value part of the outputs of the first and second absolute value detectors 3f and 3g.
For example, if the output of the first absolute value detector 3f is smaller than that of the second absolute value detector, it means that the correlation between the input video signal V and the 1 H-delayed video signal VH is larger than the correlation between the 1 H-delay video signal VH and the 2 H-delayed video signal VHH.
Then, the selection signal provided from the correlation detector 3h becomes `0` so that the multiplexer 4 selects and provides the first color signal component generated by comb-filtering of the input video signal and the 1 H-delayed video signal.
On the contrary, if the output of the first absolute value detector 3f is larger than that of the second absolute value detector 3g, the multiplexer 4 selects and provides the second color signal component by the selection signal of `1`.
Next, the BPF 104 filters the first or second color signal component to provide the complete color signal c, and the delay 104 delays the 1 H-delayed video signal to provide it in synchronization with the color signal c.
Then the subtracter 104 subtracts the color signal c from the 1 H-delayed video signal VH to select and provide only the luminance signal.
The circuit shown in FIG. 1(B) can reduce the cross color phenomenon by the adaptive 1 H comb-filtering, according to the correlation among-the three horizontal scanning lines.
But even if the three horizontal scanning lines have a larger correlations to one another, the 1 H comb-filtering is carried out to one side.
In addition, even if an irregular video input that does not agree with the assumed condition of the comb filter is applied from a video tape recorder or a game player, the 1 H comb-filtering is carried out to one side. Thus, the possibility of error generation becomes large.